1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly to reliability of a semiconductor transistor against heat.
2. The Related Art
Semiconductor transistors are used as amplifiers for mobile communication and satellite communication. Demands for higher power are increasing with regard to these semiconductor transistors. However, higher power is accompanied by increased heat generation, which can adversely influence the life of the semiconductor transistor. Some of the adverse influences that result from an increase in heat generation affect the characteristics of the semiconductor transistor and result in, for example, decreased drain current and increased gate leakage current. These not only can cause degradation in the characteristics at high temperature, but sometimes cause irreversible degradation. As a result, the device characteristics do not return to normal after the temperature falls, and may even result in massive failure or breakage. Accordingly, in making such a high power semiconductor transistor device, a technique is needed to maintain its characteristics against heat degradation.
One means for preventing transistor heat degradation is introduced in Japanese Patent Application Laid-open No. Sho 61-23350. FIG. 14 shows a sectional view of a semiconductor device (an FET substrate) described in the application No. Sho 61-23350. A cavity is provided in the rear surface of a semiconductor substrate 101. Heat generating region (active region or element formation region) 103 is formed on the front surface of the semiconductor substrate 101 and gold-plate electrode for heat radiation 102 is formed on the back surface of the semiconductor substrate 101. By making the thickness of the substrate right below the heat generating region 103, the heat radiation of the region small (thin) is enhanced.
Japanese Patent Application Laid-open No. Sho 56-131936 also proposes such a semiconductor device. FIG. 15 shows a sectional view of a semiconductor integrated circuit device described in the Application No. Sho 56-131936. A plurality of heat generating region 203 are formed on a front surface of a semiconductor substrate 201. The heat generating regions 203 are surrounded by non-generating heat regions 204. Gaps 205 are formed on a back surface of the semiconductor substrate 201 so as to oppose the respective heat generating regions 203.
However, the inventor has determined that a heat generating region of the semiconductor device has a temperature distribution depending on its shape. That is, temperature produced at a central portion of the heat generating region is high compared with temperature produced at a peripheral portion of the heat generating region. Thus, the conventional devices have the problem that device characteristics deterioration occurs at the center portion of the heat generating regions (localized hot spots) of the device due to uneven distribution in the heat generating region and concentration of the heat at the center of the heat generating portion of the device.